1. Field of the Invention
This invention relates to an illumination optical arrangement, and more particularly to an illumination optical arrangement for illuminating a specimen in a microscope or for a device for projecting onto a semiconductor substrate a pattern on a reticle in the field of a semiconductor manufacturing apparatus.
2. Description of the Prior Art
In an illumination optical arrangement using a discharge type light source such as a super-high pressure mercury lamp, it is most efficient to condense light by a concave reflecting mirror such as an elliptical reflecting mirror, but this produces an illumination light beam in which light is absent near the optical axis and the central portion of the beam is hollow. Accordingly, if such illumination system is used in a microscope or the like wherein illumination is effected as transmitted light relative to an object and imaging is effected by an objective lens, the illumination has an annular distribution and light is absent at the center of the beam passing through the entrance pupil of the objective lens. The result is that only a marginal light beam is passed through the objective lens, so that aberration is a problem, and this is not desirable.
Also, where the numerical aperture (N.A.) of the illumination light is prescribed corresponding to the numerical aperture (N.A.) of the objective lens, there has been the disadvantage that the absolute quantity of light is less because light is absent at the center of the light beam.
Usually, when EQU illumination N.A./objective N.A.=.sigma.,
if the illumination N.A. is set so that .sigma. is a value more or less smaller than 1, both depth of field and resolution of the objective lens will be at their best. In the illumination system of this type, it is necessary to place a stop in an annular portion having a great quantity of light in order to restrict the N.A. of the illumination light beam, and the quantity of light effectively used decreases. For example, a collimating lens of a short focal length may be used so as to be coincident with the set .sigma.. However, the actual light source is not a point source of light but has a finite size and therefore, if the focal length is short, the light beam which should become parallel rearwardly of the collimating lens will have an angle relative to the optical axis and will be intercepted by a stop and thus, a great quantity of light will be wasted.